Scalable Multiple-Inverse Diffusion Flame Burner for Synthesis and Processing of Carbon-Based and Other Nanostructured Materials and Films and Fuels

ABSTRACT

Apparatus and methods of use thereof for the production of carbon-based and other nanostructures, as well as fuels and reformed products, are provided.

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/446,789, filed on Feb. 25, 2011,and U.S. Provisional Patent Application No. 61/550,028, filed on Oct.21, 2011. The foregoing application is incorporated by reference herein.

This invention was made with government support under Grant No.W911NF-08-1-0417 awarded by the Army Research Office, under Grant No.N00014-08-1-1029 awarded by the Office of Naval Research, and underGrant No. CTS-0522556 awarded by the National Science Foundation. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

This invention describes apparatus and methods for the synthesis andprocessing of nanostructured materials and articles derived thereof aswell as fuel reforming.

BACKGROUND OF THE INVENTION

Carbon-based nanostructures and films define a new class of engineeredmaterials that display remarkable physical, photonic, and electronicproperties. Graphene is a monolayer of sp²-bonded carbon atoms in atwo-dimensional (2-D) structure. This layer of atoms can be wrapped into0-D fullerenes, rolled into 1-D nanotubes, or stacked as in 3-Dgraphite. Graphene and carbon nanotubes (CNTs) exhibit unique electronicand photonic properties, high thermal conductivity, and exceptionalmechanical properties. Diamond, comprised of sp³-bonded carbon atoms, iswell known for its extreme hardness, high thermal conductivity, wideband gap, and large optical dispersion.

Recently, the discovery of graphene by micro-cleaving has generatedintensive experimental research into its fabrication. Production methodsthat currently exist include ultrahigh vacuum (UHV) annealing of SiC,and chemical vapor deposition (CVD). Common techniques for CNTfabrication include plasma-arc discharge, laser ablation, and CVD.Methods for the synthesis of fullerenes include electrode-arc processes.

Although these methods have met with some success, they are not readilyor economically scalable for large-area applications or may be subjectto batch-to-batch inconsistencies. Combustion synthesis has demonstrateda history of scalability and offers the potential for high-volumecontinuous production at reduced costs. In utilizing combustion, aportion of the hydrocarbon gas provides the elevated temperaturesrequired, with the remaining fuel serving as the hydrocarbon reagent,thereby constituting an efficient source of both energy and hydrocarbonreactant. This can be especially important as the operating costs forproducing advanced materials, especially in the semiconductor industry,far exceed the equipment costs. Various morphologies of CNTs, carbides,and semiconducting metal-oxide and carbide nanowires have been producedusing air-fuel combustion-based configurations, using both aerosol andsupported-substrate methods.

The growth of these nanostructures and films over large areas remainsespecially challenging. Moreover, current processing methods can becomplex, while still characterized by low growth rates and low totalyield densities. Accordingly, it is evident that there is a strong needfor better methods of synthesizing nanostructures, particularlycarbon-based nanostructures.

SUMMARY OF THE INVENTION

Apparatus and methods for processing nanostructures (e.g., carbon-basedand other oxide, carbide, nitride, boride, and silicide phases, ormixtures thereof) and articles derived therefrom are provided. Theapparatus can also be used for fuel reforming (e.g., hydrogen or syngasgeneration from natural gas).

In a particular embodiment, the method comprises reacting an oxidizerand a fuel in a non-premixed, multiple, inverse-diffusion-flame burner.The non-premixed, multiple, inverse-diffusion-flame burner may comprisean array of stabilized flames that form a uniform flat-flame front.

In a particular embodiment, the pyrolysis species exiting thenon-premixed, multiple, inverse-diffusion-flame burner are directed ontosubstrates or particles to form films, coatings, and preforms. Inanother embodiment, the pyrolyzed species exiting the non-premixed,multiple, inverse-diffusion-flame burner are quenched to generatenanostructured particles by a vapor condensation mechanism. Thepyrolyzed species exiting the non-premixed, multiple,inverse-diffusion-flame burner may also be used to infiltrate a porouspreform. In a particular embodiment of the instant invention, thepyrolyzed species exiting the non-premixed, multiple,inverse-diffusion-flame burner provides heating, fluidization, andprecursor loading for micron-size particles. The pyrolyzed speciesexiting the non-premixed, multiple, inverse-diffusion-flame burner mayalso be used to provide heating, levitation, and coating of flatsubstrates.

In accordance with another aspect of the instant invention, methods ofsynthesizing molecular hydrogen or a syngas are provided. In aparticular embodiment, the molecular hydrogen or a syngas is synthesizedby reacting an oxidizer and a fuel in a non-premixed, multiple,inverse-diffusion-flame burner.

In accordance with another aspect of the instant invention, methods ofconverting natural gas into hydrogen and syngas are provided.

In accordance with another aspect of the instant invention, porousceramic honeycomb structures, e.g., ones that are configured forextraction of hydrogen from reformed diesel fuel and other hydrogen-richgases, are provided. Methods for extracting hydrogen are also provided.In a particular embodiment, the porous ceramic honeycomb structure isinfiltrated with a Pd alloy (e.g., a Pd/Ag alloy, more particularlyPd/20 Ag) to form a thin coating. The Pd alloy coating may be sinteredto full density in order to provide a path for selective hydrogendiffusion and efficient hydrogen extraction and/or a path for hydrogendiffusion through the nanograined structure and along its grainboundaries. The Pd alloy coating may have a nanocrystalline structure,thereby enhancing hydrogen permeability by grain boundary diffusion. Ina particular embodiment, the porous ceramic honeycomb structure iscoated by a dip coating procedure with the application of pressure toensure infiltration into the open porous structure. The Pd alloy coating(e.g., a nanocrystalline Pd/20 Ag membrane) may also serve to enhancethe strength of the alloy without compromising toughness.

In accordance with another aspect of the instant invention,inverse-diffusion flame (IDF) burners (e.g., ceramic burner) thatcomprise a reconfigured catalytic converter are provided. In aparticular embodiment, the reconfigured catalytic converter providesseparate feed streams for oxidizer (e.g. air or oxygen) and fuel (e.g.methane or ethane). The issuing oxidizer and fuel feed streams may beignited to form a uniformly flat flame. In a particular embodiment, theIDF burner is configured as a catalytic reactor. The catalyst materialmay be in the form of, without limitation, a thin section ofcatalyst-impregnated honeycomb structure, a catalyst-coated metal mesh,or a bed of catalyst-particle aggregates (extrudates). In a particularembodiment, the catalyst material (e.g., high-surface-area catalyst) islocated in the flame, particularly the hot zone of the flame, such thatthe resulting pyrolysis species generate, for example, various gaseousproducts, including hydrogen and syngas. Scaling of the technology maybe accomplished, for example, by laying several identical IDF burnersside-by-side to form any desired size, shape or form.

According to another aspect of the instant invention, methods forfabricating diamond-strengthened composites are provided. In aparticular embodiment, the method utilizes an inverse-diffusion flame(IDF). The method of the instant invention may comprise the steps of:depositing a very thin coat of Fe or other catalyst on a substrate(e.g., nanocrystalline) by thermal decomposition of a metalorganicprecursor (if necessary); growing a thin coat of diamond (e.g.,nanocrystalline) on the Fe-coated (or other catalyst-coated) substrateby low-temperature (e.g., <500° C.) thermal decomposition of ahydrocarbon precursor; and growing a thicker coat of diamond (e.g.,textured microcrystalline) on the thin diamond-coated substrate byhigh-temperature (˜1000° C.) thermal decomposition of a hydrocarbonprecursor. The metalorganic precursor may be a volatile Fe-rich compoundsuch as iron pentcarbonyl or ferrocene, or other element based compound.The hydrocarbon precursor may be a volatile C-rich compound such asmethane or ethylene. The process may be used to apply diamond (and otherhard materials (e.g., SiC, TiC, boron carbide (B4C), c-BN, etc.))coatings to fiber materials (e.g. C or SiC), woven-fiber materials,nano-fibrous materials, or film/sheet materials (e.g. polymer, metal, orceramic). The diamond-coated fibers may be used to fabricatefiber-reinforced polymer-matrix composites (PMCs), metal-matrixcomposites (MMCs), or ceramic-matrix composites (CMCs). Thediamond-coated film/sheet materials may be used to fabricate laminatedpolymer-, metal-, or ceramic-matrix composites. Woven-fiber materialsmay be infiltrated with diamond to form rigid diamond-reinforcedcomposites that contain residual porosity or infiltrated with diamond byvarying the gas flow rate (residence time) to obtain uniformthrough-thickness deposition. In a particular embodiment, when thecomposite comprises residual porosity, the pores are filled bypressure-infiltration of a compatible liquid phase (e.g. Si or Ti).

In a particular embodiment, carbon nanotubes (CNTs) or silicon-carbidenanotubes (SiCNTs) are coated with thin/thick films of diamond. Thediamond-coated CNTs or SiCNTs may be used to fabricate D/CNT-reinforcedPMCs or D/SiCNT-reinforced CMCs. In a particular embodiment, the CNTcomponent of the diamond-coated CNT is removed (e.g., by selectivegasification in a hydrogen-rich gas stream), thereby forming diamondnanotubes (DNTs). The DNTs may be used to reinforce PMCs or CMCs.

BRIEF DESCRIPTIONS OF THE DRAWING

FIGS. 1A-1C provide schematics of a multiple inverse-diffusion flamereactor for synthesizing nanostructured carbon-based materials. FIG. 1Dshows a staged burner with two levels of multiple inverse-diffusionflames. FIG. 1E shows a staged burner where the second stage can beinert, dopant, or other reactant.

FIG. 2 provides schematics of a flame-condensation reactor (FIG. 2A), aflame-infiltration reactor (FIG. 2B), a flame-fluidization reactor (FIG.2C), a flame-levitation reactor (FIG. 2D), and a flame-reforming reactor(FIG. 2E).

FIG. 3 provides an image of multiple inverse diffusion flames in anethylene-rich environment.

FIG. 4 provides images of CNTs grown on a Fe substrate.

FIG. 5 provides images of CNTs grown on a Ni/Cr/Fe substrate.

FIG. 6 provides an image of CNTs grown on a Ni/Ti substrate.

FIG. 7 provides an image of CNTs grown on a Ni substrate.

FIG. 8 provides an image of multiple inverse diffusion flames in ahydrogen/ethylene rich environment.

FIG. 9A shows graphene layers grown on nickel. FIG. 9B shows the Ramanspectra, with 514 nm excitation, from the sample, showing the presenceof 5 to 8 layers of graphene.

FIG. 10A shows graphene layers grown on copper. FIG. 10B shows Ramanspectra, with 633 nm excitation, from the sample, showing the presenceof 5 to 8 layers of grapheme.

FIG. 11 provides a schematic of an alternate arrangement of the setupfor CVD-type growth.

FIG. 12A provides a schematic of a ceramic honeycomb structure, modifiedto satisfy the requirements of an inverse-diffusion flame (IDF) burner,showing separate chambers for delivery of fuel and air to an array oftiny flames. FIG. 12B provides a schematic of an IDF burner,reconfigured as a catalytic reactor, showing location of catalystmaterial in the hot zone of the flame.

FIG. 13 provides a schematic of a design for a hydrogen extractor, withinset showing diffusion of hydrogen through a thin Pd/Ag membrane thatis supported on a ceramic honeycomb structure.

FIG. 14A provides a graph of the temperature dependence of carbonactivity (a_(c)) for diamond and Fe₃C (cementite), showing stability ofdiamond at 840-855° K (˜580° C.). FIG. 14B provides a metastable Fe—Cphase diagram, showing α+C_(dia) at <580° C.

FIG. 15 provides a schematic of a pultrusion process, showing locationof IDF coaters.

FIG. 16 provides a schematic of a layer-deposition process using an IDFburner.

DETAILED DESCRIPTION OF THE INVENTION

Flame synthesis research may be directed at growing nanostructures andfilms on substrates, at high growth rates, and in open environments suchas ambient air. The instant invention, which utilizes multipleinverse-diffusion (non-premixed) flames, not only achieves the aspectsmentioned hereinabove, but also allows for the manufacture ofnanostructured powders and films reliably and consistently withspecified properties, morphologies, and lay-out architectures for deviceapplications.

The instant invention describes apparatus and methods for the synthesisand processing of nanostructured materials and articles derived thereof.In a particular embodiment, the apparatus consists of a multipleinverse-diffusion (non-premixed) burner, where for each tiny stabilizedflame, the oxidizer-feed tube is in the center and an array of fuel-feedtubes surrounds it. The method may involve quenching pyrolyzed speciesto form nanostructured particulates or depositing pyrolyzed species ontoa heated substrate to form nanostructured films, fibers, or coatings.Using various hydrocarbons as fuels, the method may be used to generatecarbon-based materials, such as fullerene particles, carbon nanotubes,and graphene sheets. Additions of other precursors to the hydrocarbonand/or hydrogen fuel enable the processing of, e.g., nanostructuredoxide, carbide, nitride, boride, and silicide phases, or mixturesthereof. The burner is capable of continuous operation in open andclosed environments, and can also be used as a hydrogen or syngas(synthetic gas) generator.

In a particular embodiment of the instant invention, a multipleinverse-diffusion (non-premixed) burner (see, e.g., FIG. 1) is utilizedwhere for each tiny stabilized flame, the oxidizer (e.g., air) is in thecenter and fuel (e.g. ethylene) surrounds it. Multi-element diffusionflame burners (though not an inverse-diffusion burner) are described,for example, in U.S. Patent Application No. 2004/0050207.

The instant invention encompasses the synthesis of nanostructures (e.g.,fibers, films, discs, plates, sheets, flakes, carbon onions, granules,particles, nanotubes, nanowires, etc.), particularly carbon-basednanostructures (e.g., graphene sheets/flakes, carbon nanotubes, andfullerenes), in which the reagents are produced by the combustionprocess itself. In terms of the fundamental physio-chemical routesinvolved, the key precursor/reagent species do not have to pass throughthe reaction zone, affording the synthesis of non-oxide ceramics. Evenin the synthesis of oxides, the H₂O and CO₂ pathways are isolated(greatly impacting morphology) versus the O₂ route specified in PatentApplication No. 2004/0050207.

The present invention also encompasses the direct synthesis ofnanostructures and films on substrates (solid or liquid) and particlesin a readily scalable manner, where either the substrate or the burneritself is moveable via translation or rotation. Pre- or post-treatmentof the substrate is also provided by the same burner in an uninterruptedmanner, and functionally-graded films can be produced during thesynthesis process. External forces (e.g., electromagnetic) can also beapplied to yield specific particle morphologies. Additional details ofthe capabilities and advantages of the invention are describedhereinbelow.

As explained in more detail hereinbelow, the instant inventionencompasses apparatus and methods for producing nanostructures (such ascarbon-based and ceramic (non-oxide and oxide) nanostructures) utilizinga multiple inverse-diffusion (non-premixed) burner. In a particularembodiment, the non-premixed burner comprises an array of tinystabilized flames that form a uniform flat-flame front. The stabilizedflame of the multiple-flame burner may comprise an oxidizer-feed tube inthe center and fuel-feed tubes surrounding it. In one embodiment, theoxidizer is air, O₂, or any other reducing agent (e.g., fluorine orbromine). The fuel may be a hydrocarbon (e.g., CH₄, C₂H₄, C₂H₂) or anyother reagent (e.g., hydrogen, hydrazine, alcohol, acid, etc.). H₂ fuelsmay contain other reactants, thus yielding nanostructured carbides,borides, nitrides, and other phases or mixtures thereof. In a particularembodiment, the fuel is H₂+SiH₄+CH₄ or H₂+SiH₄+NH₃, thus yieldingnanostructured SiC or Si₃N₄ particles, fibers, or films.

In a particular embodiment, the post-flame species are largely composedof pyrolysis vapors that have not passed through the oxidation orcombustion zone (thus, effectively separating reduction processes fromoxidation processes). The pyrolysis vapors may be directed onto asubstrate (optionally heated (e.g., moderately)) to form nanostructures.The pyrolysis vapors may contain additives including, withoutlimitation: N-species (e.g., NH₃) or B-species (e.g., BH₃), to formdoped materials. The dopant concentrations may be sufficiently high toform nanostructured phases (e.g., C₃N₄, B₄C and BN).

In a particular embodiment, each stabilized flame of the burner is about1 mm in diameter. In yet another embodiment, the oxidizer/fuel ratio isadjusted to obtain a desired flame temperature, for example: <400° C.(cool flame mode), 800-1400° C., or about 1000° C. The multiple-flameburner may be operated in an ambient-air environment, particularly usingan inert-gas (e.g., N₂, Ar, He) or tubular metal or ceramic shielding.

The substrate may be liquid or solid. In a particular embodiment, thesolid substrate is metallic or ceramic. Examples of solid substratesinclude, without limitation: copper, alumina, silicon carbide, aluminumnitride, diamond, and the like. In one embodiment, the liquid substratemay be a low melting point metal (e.g., tin or tin-coated copper) or lowmelting point ceramic (e.g., borosilicate glass). The substrate may alsobe a thermoplastic or thermosetting resin (e.g., polyacrylonitrile,polymethylmethacrylate, or epoxy). The deposit/substrate interface maybe functionally-graded by adjusting fuel composition and flow rates orintroduction of precursors for doping.

In another embodiment, the gas velocities (and corresponding flow rates)may be adjusted to achieve specific production rates. Additionally,diverging or converging nozzles can be incorporated into the burner inthe post-flame region to tune the velocities at the substrate orparticles. The gas flow velocity may be independent of burner diameterand where temperature and chemical species are radially uniform, thusensuring uniform deposition rates over large areas.

Either the substrate or the burner itself may be moveable (e.g., viatranslation or rotation), in order to enable continuous production ofcarbon-based or ceramic (oxide and non-oxide) nanostructures.Additionally, the substrate surface may be pre-treated. In a particularembodiment, the pre-treatment of the substrate surface is provided bythe same burner, e.g. for de-scaling of metals by hydrogen reduction.

In a particular embodiment, the pyrolysis vapors exiting themultiple-flame burner are directed onto substrates or particles to formnanostructured coatings (e.g., fibers or films, thereby forming aflame-deposition reactor). The flame-deposition reactor may be used tobuild up incrementally a nanostructured deposit on a rotating mandrel(e.g., a rotating rod or cylinder).

In yet another embodiment, the pyrolyzed species exiting themultiple-flame burner are rapidly quenched to generate nanostructuredparticles by a vapor condensation mechanism, thus forming aflame-condensation reactor. The rapid quenching of a hot gas streamconsisting of pyrolyzed hydrocarbon species (e.g., from CH₄ or C₂H₄) maybe used to generate carbon nanoparticles (e.g., fullerenes, carbononions, or graphene flakes). In another embodiment, the rapid quenchingof a hot gas stream consisting of a mixture of pyrolyzed hydrocarbon andother reactive species (e.g., from BH₃, H₃NBH₃, SiH₄ or (CH₃)₃SiH) maybe used to generate carbon nanoparticles that are enriched in B, N, Si,or mixtures thereof. In yet another embodiment, the rapid quenching of ahot gas stream consisting of pyrolyzed metalorganic or organometallicprecursors is used to generate nanoparticles of carbides, borides,nitrides, silicides, or mixtures thereof. The rapid quenching may beaccomplished by directing the hot gas stream onto a chilled metalsubstrate (e.g., Cu or Al) (e.g., rotating copper wheel or drum), into across-flow of cooling gas (e.g., Ar or N₂), or into a liquid medium(e.g., water or liquid N₂). The as-synthesized nanoparticles may have anarrow particle size distribution. The as-synthesized nanoparticles maybe loosely agglomerated or aggregated into larger particles (e.g.,submicron-sized) with open or closed nanopores. In another embodiment,multiple stages of multiple inverse-diffusion flames can be formed byextending certain tubes. FIG. 1D shows two levels of flames, howevermultiple levels can be established. Such staging allows for tuning ofresidence time and temperature histories for the reactants and pyrolysisspecies.

In another embodiment, inert, dopant, or other reactant can beintroduced at another level(s) (see FIG. 1E). Introduction of inert(s)can facilitate nanoparticle formation or quenching of specificreactions. Introduction of dopant(s) at a different level allows tuningof residence time for the decomposition kinetics, especially if multipledopants are simultaneously injected. Similarly, reactant(s), such asmethane, can be introduced at another level to achieve specificdecomposition kinetics affecting synthesis conditions, for example, ingrowing monolayer graphene.

In another embodiment, the gas stream exiting the multiple-flame burneris used to infiltrate a porous preform with another phase, thus forminga flame-infiltration reactor. In a particular embodiment, the porouspreform comprises aligned or woven carbon fiber. The infiltrationcomponent (infiltrant) may be graphene-like carbon, thus forming a C/Ccomposite. In a particular embodiment, the infiltration is carried outwith the fibrous structure under a temperature gradient to promoteuniform deposition, leaving minimal open porosity in the C/C composite.In yet another embodiment, catalytic growth of CNTs within the porespace of the carbon-fiber preform precedes final infiltration withgraphene-like material, thus forming a nano/micro C/C composite. Anarray of CNTs growing on a substrate may also be simultaneouslyinfiltrated with graphene-like material, thus forming atransverse-reinforced C/C composite. An array of CNTs grown on asubstrate may also be flattened (e.g., by rolling), prior toinfiltration with graphene-like material, thus forming alongitudinal-reinforced C/C composite.

According to another embodiment, the gas stream exiting themultiple-flame burner provides heating, fluidization, and precursorloading (e.g., carbon-based species or other metalorganics) formicron-size particles, thus forming a flame-fluidization reactor. Thepyrolysis species in the hot gas stream may uniformly coat the fluidizedbed of particles with nanostructured particles, fibers, or films. Bychanging the composition of the flame-precursor mixture, the fluidizedbed of particles may be uniformly coated with nanostructured core-shellor multi-layered structures. In a particular embodiment, the bedtemperature is made more uniform by external heating of the reactorwall. The flow rate of the gas stream may be adjusted to ensure uniformfluidization of the powder bed. A blow-back filter may also be used toprevent the fluidized particles from escaping into the environment,while facilitating disposal of exhaust gases.

In another embodiment, the gas stream exiting the multiple-flame burnerprovides heating, levitation, and coating of flat substrates, thusforming a flame-levitation reactor. The substrates themselves cantranslate and/or rotate effortlessly in a continuous coating productionmode. In one embodiment, the pyrolysis vapors in the hot gas streamuniformly coat flat substrates with nanostructured particles, fibers, orfilms. The reactor may direct the gas stream onto a moving belt, therebyenabling continuous fabrication of coated substrates. The sequence ofsuch burners, utilizing different precursor materials, enables thecontinuous production of multi-layered, doped, or graded nanostructures.

As explained in more detail herein, the nanostructures produced by theapparatus and methods of the instant invention have many uses. Forexample, the flame-deposition reactor may be used to fabricatelarge-area films of graphene (single- or multi-layer) or large-areadeposits (unidirectional or random weave) of CNTs or nanofibers(metallic, oxide or carbide) or large area diamond films. The graphenefilms (continuous or patterned) may then be used for electronic,photonic, and other applications. The CNT deposits may be used tofabricate C/C composites and polymer-matrix composites. The diamondfilms may be used for superhard coatings and corrosion resistance. Theflame-deposition reactor may be also used to grow graphene on a templateof metal (e.g., Cu) nanowires to form nano-ribbon like structures orused to sublimate Si from a SiC substrate and grow graphene in ahydrocarbon-rich environment.

In addition, the flame-condensation reactor may be used to produceloosely-agglomerated or aggregated carbon nanoparticles for electronic,photonic and other applications. In particular, the loosely-agglomeratedgraphene flakes (doped and undoped) may be dispersed in a fluid mediumto produce an ink for use in fabricating interconnects in printedcircuit boards and electronic devices. The open nanopores of aggregatedcarbon nanoparticles (doped or undoped) may also be surface modified toenhance catalytic and other properties.

The flame-infiltration reactor may also be used to fabricate C/Ccomposites for high temperature structural applications (e.g.,gas-turbine engines, rocket engines, and thermal protection systems) andthe flame-fluidization reactor may be used to uniformly coatmicron-sized particles or granules with nanostructured particles, films,or fibers. The films (coatings) may be superhard materials (e.g.,diamond, cubic-BN, and mixtures thereof) or hard materials (e.g.,carbides, borides, nitrides, and mixtures thereof). The coatings may beapplied to machine tools, mining tools, rock-drill bits, punch and diesets, and bearings to enhance wear performance and to increase servicelifetimes. The micron-size particles may have open-porous structures andcan be infiltrated with nanostructured particles, films, or fibers. Theopen pores of activated-carbon may also be infiltrated withhigh-surface-area CNTs to enhance electrode performance insupercapacitors and batteries. The flame-fluidization reactor may alsobe used to form powders/flakes of graphene on Cu particles/flakes andmay also be used to form powders/flakes of graphene directly from SiCparticles via sublimation and/or hydrocarbon deposition.

Additionally, the flame-levitation reactor may also be used forlarge-area fabrication of nanostructured particles, films, and fibers,with the advantage of lower production costs relative to theflame-deposition rector.

The flame-reforming reactor of the instant invention may also be used tosynthesize molecular hydrogen and/or syngas from various fuels, such asmethane, natural gas, methanol, gasoline, diesel, JP-8, and biofuels.

The multiple inverse-diffusion flame burner of the instant inventionconfigured as a flame catalytic/reforming reactor is ideally suited forapplications in which there is a need to convert natural gas into usefulproducts.

Inverse Diffusion Flames

One feature of the instant invention is the operation of a burner in themultiple-inverse diffusion (non-premixed) flame mode. Each of thediffusion flames is run in the inverse mode (“under-ventilated”), wherefor each flame the oxidizer (e.g., air or O₂) is in the center and fuel(e.g., methane, ethylene, acetylene, etc.) surrounds it. The net effectis that post-flame species are largely comprised of pyrolysis vaporsthat have not passed through the oxidation zone. In fact, the reactionzone serves as a “getterer,” such that the oxygen mol fraction isreduced to ˜10⁻⁸ in the post-flame gases.

Carbon formation processes are effectively separated from oxidationprocesses in inverse diffusion flames, which also tend to soot less thannormal diffusion flames. No soot was observed in the multiple-inversediffusion flame burner of the instant invention under the conditionsexamined. Moreover, the hydrocarbon species (e.g., rich in C_(n) andCO), which serve as reagents for synthesizing carbon-basednanostructures and films, are generated in much greater quantities thanthat achievable in stable, self-sustained premixed flames. By usingdiffusion flames (burning stoichiometrically in the reaction zone),flame speed, flashback, and cellular instabilities related to premixedflames are avoided. Additionally, unwanted oxidation of precursors (fordoping) upstream of the burner is eliminated. Since many small diffusionflames are utilized, overall radially-flat profiles of temperature andchemical species are established down-stream of the burner. Confinementin an inert environment or shielding with an inert co-flow prevents anencompassing diffusion flame to develop.

This novel, robust configuration is well suited for the growth ofgraphene, CNTs, and fullerenes on substrates, for use, e.g., in devicesor as coatings. The non-premixed flame process competes with CVD-typeprocesses in the growth of graphene and CNTs. The present setup allowsfor faster growth rates due to scalability and high flow rates ofprecursor species; better control of temperature and reagent speciesprofiles due to precise heating at the flame-front, along withself-gettering of oxygen; and reduced costs due to efficient use of fuelas both heat source and reagent.

The main hydrocarbon fuels include, without limitation, methane,ethylene, and acetylene, along with their blends. H₂O is a mainbyproduct of hydrocarbon combustion, which may result in unwantedoxidation reactions at elevated temperatures. However, introduction ofhydrogen helps to reduce any undesired oxides. Moreover, non-oxide basedreactants (e.g., H—Br system) are being investigated.

In a particular embodiment of the instant invention, the flamesoperating in a partially premixed mode can be run in a cool-flame mode,such that the maximum flame temperature is <400° C., thus permittingdeposition of materials on polymers.

Scalability

For a premixed flame burner operating in the laminar regime, to maintainthe same Reynolds (Re) number, if the burner diameter is doubled, theissuing velocity must be halved, so that the mass flow rate is only twotimes larger. Thus, the nanostructure/film production rate is doubled.However, by reducing the issuing flow velocity, the samedivergence-stabilized laminar flame cannot be established, since theflame would be stabilized further upstream where the flame speed matchesthe local flow velocity. Eventually, one would reach the limit offlashback. To circumvent flashback, a burner-stabilized flame could beused, but that could introduce clogging issues due to the use ofshower-head-type or porous burners.

Operation of a non-premixed flame burner (as in the instant invention)has no scaling problems by allowing for stability at all burnerdiameters, where the issuing flow velocity can be independent of theburner diameter. The post-flame gases downstream are quasi 1-D in thatthey are radially-uniform in temperature and chemical species. Thus,larger burners and substrates can be used, while ensuring uniform growthrates at the substrate. Additionally, the substrates can be placed on aconveyor-belt for high-production throughput or the burner can betranslated or rasterized to generate a very large area coating ordeposit.

Substrates

According to one aspect of the instant invention, nanostructures andfilms can be grown on substrates. The substrates can be of variouscompositions (e.g., metal, non-metal, mixed) and either liquid or solid.Liquids can serve to provide extremely flat surfaces. Rotating thesubstrates (e.g., in spin coating) to maintain flatness and uniformityof the growth layers is readily accommodated. For example, a thin moltenlayer of tin can exist on a copper substrate, on which graphene, forexample, can be grown. Depending on the temperature that suchnanostructures and films can be deposited, the substrate may be a glassor polymer (e.g., polymethyl-methacrylate (PMMA)). Molten glass, basedon float-glass technology, can serve as a substrate. The invention isalso compatible with current technology on surface modification of plateglass using CVD. Growth on polymer substrates, such as PMMA, would besignificant in terms of applications involving transparent materials,such as interlayers in photonic, electronic, or EM-shieldingapplications. For the semiconductor industry, graphene layers couldreplace silicon or copper interconnects in devices.

In a particular embodiment, the substrate would have to be pre-treatedto remove any oxide layer, e.g. copper substrate for the growth ofgraphene. The unwanted oxide layer on the metal substrate is removed viahydrogen reduction by using only hydrogen as fuel (>1 global equivalenceratio) in the multiple inverse-diffusion flame burner. After a givenperiod, the fuel is switched over to a hydrocarbon (e.g., to growgraphene) in a continuous manner.

Alloying/doping

Alloying of nanostructures such as graphene with nitrogen and/or boron,as well as other elements, is also encompassed by the instant invention.For example, ammonia (NH₃) can be introduced with the hydrocarbon fuel(e.g. CH₄) to provide a source of nitrogen, such that the NH_(x) speciesformed during flame decomposition are incorporated directly into thegraphene structure. Similarly, borane (BH₃) or borane-ammonia (H₃NBH₃)can be introduced with the hydrocarbon fuel to provide a source of boronand/or nitrogen. It is possible that the hexagonal symmetry of thegraphene can be retained, while incorporating boron or boron nitrideinto the structure. Additionally, halogenation can be applied toincorporate fluorine and other elements into the structure using theappropriate precursors. Properties of the nanostructures may be altered.

The final coating/film may also be nitridized, boronized, carbonized,etc., after the main synthesis phase. With water as a key productspecies, oxidation of the films (in varying degrees) is possible bycontrolling the temperature and H₂ concentration. In addition, acoating/film can be functionally-graded (in composition and density) byadjusting fuel composition and flow rates (or introduction of precursorsfor doping) during the synthesis process. Composites andheterostructures are envisioned (e.g. diamond-graphene-CNT).

External Field Application

Recognizing that various parameters affect graphene and CNT properties,the additional degrees of freedom via process inputs from combustiondynamics and electromagnetic field influence become exceptionallyuseful. Preliminary experiments have shown dramatic increases in theyield of CNTs when electric fields are present. Furthermore, voltagebias has been shown to promote formation of coiled nanotubes, perhapsthrough altering of local chemical species distributions.

The setup for the instant invention readily permits external electricfield application, as well as voltage bias imposed on the substrate,especially if it is metallic (e.g., nickel or copper). The stagnationplane (substrate) that serves as the location for graphene and CNTgrowth can maintain a voltage bias, which also dictates the overalluniform electric field.

Plasma-assisted combustion (e.g., where an RF-coil encompasses theburner exit region) can aid the synthesis process. Thermal-plasmaassistance can raise the gas-phase temperature to alter chemical routesand/or pyrolyze low-vapor-pressure precursors or dopants.Non-equilibrium-plasma assistance can crack specific fuels (e.g.,generate hydrogen from hydrocarbons) or pyrolyze precursors/dopants orproduce chemical radicals to alter the properties of the as-synthesizedmaterials.

Ceramic Burner and Catalytic Converter

A typical automobile catalytic converter comprises three maincomponents: (1) an extruded ceramic honeycomb core to provide highsurface area of contact with the exhaust gas stream; (2) a wash-coat offine silica and alumina particles to increase the surface area of theotherwise relatively flat core; and (3) an exceptionally high surfacearea catalyst, such as Pt, Pd or Rh, applied with the wash coat topromote desirable surface reactions. In service, catalytic convertershave proved to be mechanically robust, long lasting, and thermal-shockresistant. The latter effect is a consequence of careful texturing ofthe extruded ceramic. Millions of units have been installed inautomobiles and trucks to clean up exhaust systems. Herein, it isdescribed how catalytic converters (e.g., those manufactured by CorningInc.) can be modified to satisfy the requirements of aninverse-diffusion flame burner, while providing lower fabrication costsand greater operational flexibility. In addition, it is described how aporous ceramic honeycomb structure can be configured for extraction ofhydrogen from reformed diesel fuel or other hydrogen-rich sources. Theopenings of the honeycomb structures of the instant invention can haveany shape (e.g., circle, triangle, square, hexagon, pentagon, octagon,irregular, etc.).

In accordance with the instant invention, a conventional catalyticconverter can be reconfigured to satisfy the requirements of independentfuel- and oxidizer-feed streams for an IDF burner. As illustrated inFIG. 12A, the lower half of an extruded honeycomb ceramic is cut back toexpose oxidizer-feed tubes, which are inserted into holes drilled into aflat plate and sealed with heat-resistant cement, thus forming afuel-feed chamber. The lower chamber is then closed off to form anair-feed chamber. In a particular embodiment, the multi-tubular ceramicis composed of a uniform array of feed tubes, each <1 mm dia. Thus, theresulting flame is uniformly flat over the entire burner surface. Inpractice, the flow rates of the separate feed streams are adjusted toachieve the desired combustion conditions, such that a high density ofpyrolysis species or free radicals is generated in the combustion zone,as required to form nanostructured particles by quenching the hot gasstream or to form films, coatings, fibers, and free-standing forms bydeposition on a heated substrate.

This same ceramic burner may also be configured as a catalytic reactor,FIG. 12B. For example, a thin section of porous honeycomb structure isimpregnated with a high-surface-area catalyst and placed in the burnerhot zone to promote desired chemical reactions. Alternatively, acatalyst-coated metal mesh or a bed of particle aggregates (extrudates)is used for the same purpose. Interaction between the hot gas streamexiting the burner and the catalyst material generates various gaseousproducts, depending on the composition of the catalyst material. Forexample, using methane as precursor feed, IDF-induced pyrolysis in thepresence of an appropriate catalyst may generate a liquid fuel (mixtureof high molecular weight hydrocarbons). Using specialty catalysts, highyield and selectivity of various hydrocarbon products are alsoachievable. For production purposes, scaling an IDF burner is important.With this particular burner design, scalability is readily accomplishedby laying side-by-side several (e.g., identical) honeycomb structures(square or hexagon shaped) to form any desired burner size, shape orform.

Ceramic honeycomb structures with walls that that contain a highfraction of open or interconnected porosity may be used (see, e.g.,Corning). These micro-porous ceramics can be adapted for efficientextraction of hydrogen from reformed diesel fuel, and otherhydrogen-rich gases. In the context of the present invention, thehydrogen extractor provides a useful adjunct to IDF-burner technology.

Pd/Ag alloys display significantly enhanced hydrogen permeabilityrelative to Pd. Undesirable phase transitions encountered in cycling Pdin hydrogen are suppressed. Further, Pd/20 Ag offers superior membraneperformance. The fabrication of a nanocrystalline Pd/20 Ag membrane canfurther enhance hydrogen permeability, due to the presence of relativelyeasy diffusion paths along grain boundaries. The nanocrystalline statecan also enhance the strength of the alloy, without compromisingtoughness.

FIG. 13 shows a design for a ceramic-supported nanocrystalline Pd/20 Agmembrane. A thin layer of mixed or alloyed nanoparticles is incorporatedinto the ceramic (e.g., by a simple dip-coating procedure), particularlywith the application of pressure to ensure infiltration into the openporosity. Both sides of the tubular ceramic may be infiltratedsimultaneously, thus forming finger-like networks throughout theceramic. To ensure an impervious deposit, two, three, or more suchdip-coating operations may be performed, with optional intermediatesintering treatments to achieve densification. Thus, any desiredthickness of high surface area Pd/20 Ag can be fabricated on asupporting ceramic substrate.

In the design depicted in FIG. 13, hydrogen in reformed diesel fuel, forexample, flows through one set of tubes, dissociates on the surfaces ofthe catalyst particles, and then diffuses to an adjacent set of tubes,where the purified hydrogen is extracted. The diffusion process isdriven by the concentration gradient in hydrogen. Various combinationsof materials may be used to provide high hydrogen permeability, hydrogenselectivity, temperature stability, corrosion resistance, and structuralintegrity. A manifold system at the entrance and exit of the extractorallow reformed diesel fuel to flow in and hydrogen gas to flow out.

Because of the exceptional thermal shock resistance displayed by thetextured-ceramic substrate, the hot gas stream can be taken directlyfrom the reformer to achieve the desired operating temperature.

Multiple-inverse-flame Reactor Systems

Herein, examples are provided of multiple-inverse-flame processing ofnanostructured carbon-based materials and ceramics (non-oxide andoxide), particularly carbon-based materials. In addition, descriptionsof several reactor systems are given: (1) flame-deposition reactor; (2)flame-condensation reactor; (3) flame-infiltration reactor; (4)flame-fluidization reactor; (5) flame-levitation reactor; and (6)flame-reforming reactor. the case of carbon-based materials, thedescription generally describes the processing of fullerene-likeparticles, carbon nanotubes (CNTs), and graphene films/flakes.Applications for these and other nanostructured ceramic (non-oxide andoxide) materials are encompassed by the instant invention. For example,the incorporation of CNTs in polymer-matrix composites (PMCs) andcarbon-matrix composites (CMCs) is discussed.

(1) Flame-deposition Reactor

To fabricate large-area films and coatings of nanostructured carbon, thereactor design depicted in FIG. 1 (called a flame-deposition reactor(FDR)) can be used. In a typical operation, a hydrogen-rich flame heatsa flat-metal substrate to a steady-state surface temperature, whileremoving any oxide scale on its surface. The flame composition is thenadjusted by introducing a hydrocarbon fuel (e.g., CH₄ or C₂H₄), therebygenerating a high concentration of active intermediates (e.g. C_(n) orCH_(n) species) in the post-flame gas stream. Upon making contact withthe heated substrate, the carbon-rich species react to form carbonnanotubes when the substrate is catalytic (e.g., Ni) or graphene filmswhen the substrate is non-catalytic (e.g., Cu). Both solid and liquidsubstrates may be used, but a liquid substrate (e.g., Sn) may bepreferred to facilitate removal of the deposited material afterprocessing. Liquid Sn also offers other advantages: low volatility overa wide temperature range; ease of maintenance of a clean surface fordeposition; and minimal reaction with the carbon deposit—there are noknown metal-carbide phases.

In the case of graphene deposition on a solid-Cu substrate, the graphenefilm may be removed by selective dissolution of the substrate or byetching away the deposit/substrate interface. On the other hand, whenthe graphene is deposited on a liquid-Sn substrate, then the film may beremoved mechanically, as in the familiar float-glass process. Bymodifying deposition parameters (e.g., by increasing deposition rateand/or lowering substrate temperature) graphene in the form of discretenano-sized platelets may be formed. Applications for platelet- andfilm-graphene include flat-panel displays and integrated circuits. Inparticular, formulation of a “graphene ink” seems a particularlyattractive proposition, since it would enable direct writing ofinterconnects (e.g. for integrated circuits and printed circuit boards).Large-area graphene films may also be patterned using laser ablationtechniques.

In the case of CNT deposition on a solid-Ni substrate, a particularchallenge is to remove the material after processing. Selectivedissolution is one option; another is to use tape (e.g., Scotch® Tape).After removal, the CNTs may be randomly dispersed in a polymer matrixphase (actually a monomer that is subsequently cured) to form anisotropically-reinforced PMC. Alternatively, the CNTs are flattened byrolling, and then pressure-infiltrated with the polymer matrix phase toform a unidirectionally-reinforced PMC. In a production operation, therolling step may be integrated into a well-established pultrusion orpre-preg process. Conventional C-fiber reinforced PMCs are used today ina host of structural applications (e.g., aircraft and space vehicles),where their high specific strengths and woven-fiber designs areexploited to advantage. This new class of CNT-reinforced PMCs furtherenhances mechanical performance in these and similar structuralapplications.

(2) Flame-condensation Reactor

To fabricate nanopowders of carbon-based and other non-oxide ceramics,the reactor design depicted in FIG. 2A, called a flame-condensationreactor (FCR), may be used. In this process, gaseous precursors may befed into the multiple-flame burner under conditions that generate a highconcentration of reactive intermediates in the ensuing gas stream. Togenerate nanoparticles, the gas stream may be rapidly quenched, forexample: (1) on a chilled metal substrate (e.g., rotating copper wheelor drum); (2) using a cross-flow of cooling gas (e.g., Ar or N₂); or (3)by injecting the gas stream directly into a liquid medium (e.g., wateror liquid-N₂). In all three cases, the high quenching rate inducesprolific nucleation of nanoparticles while minimizing growth. Ingeneral, the as-synthesized nanoparticles (formed by coalescence of tinyclusters) are loosely agglomerated and have a narrow particle-sizedistribution. However, quenching the gas stream in a liquid-N₂ bath maybe preferred, owing to the ease with which the nanoparticles can becollected—their separation occurs quite naturally as the liquid N₂evaporates. Quenching in water is also convenient, at least when thenanoparticles are non-reactive or hydrophobic; otherwise post-annealingis necessary to restore the nanoparticles to a pristine condition.

Using a gaseous hydrocarbon feed (e.g. CH₄ or C₂H₄), rapid quenching ofthe pyrolysis vapors yields loosely-agglomerated carbon nanoparticles,which may be in the form of fullerenes, carbon onions, or graphene-likeplatelets (flakes), depending on the specifics of the processingparameters used. Adding another gaseous precursor to the hydrocarbonfeed (e.g. BH₃ or H₃NBH₃) is a means to generate C-rich nanopowders,containing B and/or N in solid solution, with unknown consequences withrespect to nanoparticle morphology and intrinsic properties. Similarly,doping fullerenes with Si can be accomplished by adding SiH₄ or(CH₃)₃SiH to the hydrocarbon feed. By appropriate selection ofmetalorganic or organometallic precursors, nanopowders of carbide,boride, nitride, and silicide phases, or mixtures thereof, can also beproduced.

Since the multiple-flame burner is readily scalable, the FCR process maybe used for high rate production of nanopowders of a wide range ofnon-oxide ceramics, particularly carbon-based systems, and in acost-effective manner.

(3) Flame-infiltration Reactor

To fabricate CMC preforms, the reactor design depicted in FIG. 2B,called a flame-infitration reactor (FIR), may be used. In thisembodiment, a preform of PAN-derived carbon fibers, woven as needed tosatisfy final structural-design requirements, may be infiltrated withgraphene-like carbon using a multiple-inverse-diffusion-flame burner.This may be accomplished in a one-step FIR process, such that thegraphene-like carbon is deposited uniformly throughout the entire wovenstructure. Alternatively, a two-step FIR process may be used, where thegrowth of CNTs within the open pores of the preform precedes finalinfiltration with graphene-like material. Both types of CMC displayexceptional mechanical properties, but it appears that the hybridmicro/nano-CMC may offer advantages in particularly demandingapplications (e.g., aircraft-brake hubs, and thermal protectionsystems).

Such CMC composites (also known as C/C composites) are typicallyfabricated by chemical vapor infiltration (CVI). Careful control ofprocessing parameters is needed to ensure uniform deposition throughoutthe preform (e.g., temperature gradient across the preform), as the poresize diminishes during deposition. Similar control of depositionparameters is needed when using the FIR process, but there areadvantages in terms of higher deposition rates, as well as the abilityto conduct the operation in an open environment, although a closedenvironment may be preferred in some situations. There is also anopportunity to use a high precursor-flow rate, thus enabling effectivepenetration of thick sections. A burner with an array of supersonicmicro-nozzles also permits efficient infiltration at high depositionrates.

The FIR process may also be used to fabricate thin/thick sheets ofCNT-reinforced CMCs. This may be done by making adjustments to thedeposition parameters, such that an array of CNTs continues to grow(grass-like) while being infiltrated with graphene-like material. Thenet effect is the ability to generate a transverse-reinforced C/Ccomposite. Alternatively, the as-synthesized CNTs may be flattened byrolling, and then subjected to a post-infiltration treatment withgraphene-like material to form a longitudinal-reinforced C/C composite.In other words, nanostructured CMC sheets can be fabricated in which thereinforcing CNTs are either aligned perpendicular to the sheet surfaceor parallel to the sheet surface. This enables the design of CMCs for ahost of high temperature structural applications (e.g., in gas-turbineengines, rocket engines, and thermal protection systems).

(4) Flame-fluidization Reactor

To fabricate coated powders, the reactor design depicted in FIG. 2C,called a flame-fluidization reactor (FFR), may be used. In onearrangement, a heat-resistant ceramic tube is supported on amultiple-flame burner. Thus, the burner provides heating andfluidization, while also generating pyrolysis vapors that form coatingson the fluidized particles. It is noteworthy that rapid heat transferoccurs in the dynamic environment of a fluidized bed, so that uniformtemperatures and gas/solid reaction kinetics are established throughoutthe powder bed. By introducing chemical precursors along with thefluidizing gas stream, all the particles in the bed become coated via avapor-deposition process. Fluidized-bed CVD has been discussed in theliterature, but not using an FFR process.

In one example, activated-carbon granules (e.g., 10-100 μm particlesize) form a powder bed, and operating parameters (e.g., temperature,gas-phase residence time, and precursor composition) are adjusted todeposit carbon nanotubes (CNTs) within the open pores of the particles.To promote growth of single or multi-wall CNTs, the activated-carbonpowder is pre-treated with a trace amount of a potent catalyst, such asnickel or its alloys. A similar arrangement, but without the catalystmaterial, is used to infiltrate porous activated-carbon granules withgraphene platelets. Such high-surface-area carbon powders may be used tofabricate electrodes for high performance supercapacitors and batteries.

In another example, flame-fluidization of oxide or non-oxide particles(e.g. Al₂O₃ or SiC), provides inert substrates on which to depositultra-fine catalyst particles (e.g. Pt or Ni), for use in hydrogenationand hydrocarbon conversion processes. The ability to regenerate catalystmaterials, using the FFR process, offers significant economic benefits.This same reactor configuration may also be used to fabricate powderswith core-shell or multi-layer structures (e.g., SiC-coated Al₂O₃ ordiamond-coated SiC). Deposition rates using FFR technology are at leastan order of magnitude higher than CVD technology (due to characteristicflow velocities that are 1 to 2 orders of magnitude higher), whichtranslates into an economic benefit. Diamond grits of various grades,manufactured by high pressure/high temperature technology, are widelyused for grinding and polishing purposes. Diamond-coated SiC gritsproduced by FFR technology offer a cheaper alternative.

In yet another example, FFR technology may be used to applyshape-conformal coatings to finished parts, whatever their size orshape. Here again, it is the uniformity in temperature and gas/particlesreaction kinetics, characteristic of the dynamic environment of afluidized bed, which ensures uniform coating of all exposed surfaces ofthe parts. Internal cavities in the parts may also be coated, providedthat the gas stream and fluidized particles have access to the interiorsurfaces. Both overlay and diffusion coatings may be produced, dependingprimarily on reactor temperature and gas-phase residence time.

(5) Flame-levitation Reactor

To fabricate thin/thick deposits on flat substrates (e.g., discs, platesor sheets), the reactor depicted in FIG. 2D, called a flame-levitationreactor (FLR), may be used. The hot gas stream emerging from themultiple-flame burner heats and levitates the substrate, while alsodepositing material via reaction of the pyrolysis vapor with thesubstrate. Using hydrocarbon fuels, large-area fabrication of CNTs andgraphene sheets on supporting substrates can be achieved. For example, a3-D network or random weave of CNTs can be deposited on a passivesubstrate, infiltrated with a polymer precursor (monomer) and cured,thus forming a CNT-reinforced PMC in a continuous operation, similar tothat discussed above. Uniformly thin films of carbides, borides,nitrides, and silicides (e.g., TiC, TiB₂, SiC, Si₃N₄, and B₄C) can alsobe processed in a similar manner. Multi-layer deposits can also befabricated using a sequence of burners that make use of differentprecursor feed materials. All of this can be accomplished in acost-effective manner, since the FLR process can be scaled formanufacturing purposes.

To apply thin/thick deposits on powder particles, a modified FLR designmay be used. Here, the powder is aerodynamically supported on amultiple-flame burner, comprising one or more channel-shaped sections(modules) of high aspect ratio (length to width ratio). With the burnerslightly tilted, powder is introduced into one end of the reactor andgradually moves down the slope at a rate determined by the applicablereaction-processing kinetics, finally exiting at the other end of thereactor. The advantage of this design, compared to a conventionalflat-bed reactor (e.g., so-called pusher-furnace reactor), is the morerapid and complete conversion of the initial powder into the end-productpowder, since the reactive gases pass through the bed rather than overit. As the exhaust gases flow downstream, they are effectively separatedfrom the reactive gases within the powder bed. Hence, the gas-phaseactivities of the reactants remain constant during powder processing.Moreover, relatively thick layers of powder are readily accommodated,since the gas permeation though the powder is not limited by thickness.In a typical operation, a steady state condition is first establishedfor the multiple flames emanating from the burner, and then the powderis introduced. This procedure avoids possible clogging of the burnertubes with feed powder. Various burner tube nozzles can be used (e.g.,divergent or constriction-expansion nozzles) to optimize powderlevitation and transport downstream.

To illustrate the utility of the FLR process, its applicability to theproduction of nano-WC/Co powder is considered. A simple two-step processmay be used. First, a precursor powder may be produced by spray dryingan aqueous solution of mixed salts (e.g., ammonium metatungstate andcobalt acetate), thus yielding a homogenous mixed-salt precursor powder,in which the W and Co are intimately mixed at the molecular level.Second, the precursor powder may be introduced into the FLR reactor,where pyrolysis, reduction, and carburization occur in a sequentialmanner, as the powder travels down the reactor. The final product isnanostructured WC/Co powder, in which the two nanophases are uniformlymixed and in intimate contact. A feature of the process is that eachnanocomposite particle, typically about 20-50 μm in size, also containsa high fraction of open nanoporosity. Such porous particles are readilybroken down by mechanical milling to about 1-2 μm, making the powderideal for post-consolidation by liquid-phase sintering, which isstandard practice in the hardmetals industry. A similar process may beused to transform bulk WC/Co (e.g., spent machine tools and rock-drillbits) into WC/Co powder. Here, it is preferred to first to powderize thebulk material by oxidation in air at 700-900° C., prior to conversionback into WC/Co powder in an FLR reactor.

Another advantage of the FLR reactor is that additional modules may beadded to the production line in order to coat the as-processed WC/Coparticles with other components or phases. For example, a nanostructuredWC/Co powder may be coated with one or more thin layers of other hardand/or superhard materials (e.g., TiC and/or diamond), thus enhancingits performance as grinding and polishing media. An FLR reactor may alsobe used to coat micron-sized particles of metals (e.g., Al, Ti or theiralloys) or ceramics (e.g., Al₂O₃, TiC or their admixtures) withnanostructured powders, fibers or films, as discussed above.

(6) Flame-reforming Reactor (e.g., Hydrogen Synthesis)

The multiple-flame burner can also be used as a flame-reforming reactor(FRR) (see, e.g., FIG. 2E), to non-catalytically convert hydrocarbonfuels into molecular hydrogen for various applications (e.g.,transportation vehicles, energy generation, and fuel cells). With thecapability to operate at very large global equivalence ratios(e.g., >4), as well as over a wide temperature range (e.g., 400-2000°C.), the FRR can pyrolyze various fuels (e.g., methane, natural gas,methanol, gasoline, diesel, JP-8, and biofuels), with or without partialoxidation, reforming them into hydrogen or syngas. The reactor can alsobe operated at elevated pressures in order to optimize the chemicalkinetics. Additionally, the reactor can either be large scale forindustrial use or mini-sized for portable application. In terms of costand flexibility of use, the FRR will out-compete current productionmethods for H₂ production from natural gas, such as catalytic partialoxidation and steam-methane reforming.

Method to Fabricate Diamond-reinforced Composites

As described herein, an inverse-diffusion flame (IDF) method tofabricate nanostructured diamond (and other hard materials (e.g., SiC,TiC, B4C, c-BN, and the like)) coatings on heated iron and steel (andother material) substrates is provided. Here, methods to utilize suchcoatings to enhance the performance of, for example, fiber-reinforced,nanofiber-reinforced, and laminated composites are provided.

An investigation of phase equilibria in the Fe—C system showed thatdiamond is more stable than Fe₃C (cementite) at ambient pressure (Zhukovand Snezhnoi (1973) Acta Met. 21:199-201). The critical temperature forthe phase transition is ˜580° C., as shown in a plot of temperaturedependence of carbon activity for diamond and Fe3C, FIG. 14A. On thebasis of this analysis and other considerations, a modified phasediagram for Fe—C was formulated (FIG. 14B).

Recent research has provided experimental confirmation of thetheoretical prediction. Using an IDF burner with methane as precursor,thin film deposition of diamond on iron and steel substrates has beenobserved at 400-500° C., but not at higher temperatures. On thecontrary, at temperatures ˜1000° C., carbon nanotubes are formed. Evenso, if complete surface coverage of the iron substrate is achieved atthe lower temperature, then continued growth at the higher temperaturesoccurs, resulting in a diamond film with a well-defined <111> texture.In other words, a very thin film of nanocrystalline diamond formed atthe lower temperature serves as a template for much faster growth ofmicrocrystalline diamond at the higher temperature. Moreover, the growthrate for the diamond coating at the higher temperature is about twoorders of magnitude faster than chemical vapor deposition. Thus, IDF canbe used for fabrication of: (1) diamond films or coatings in a one-stepoperation, yielding nanocrystalline structures; and (2) diamond films orcoatings in a two-step operation, yielding <111> texturedmicrocrystalline structures.

In the event that the substrate material is not Fe or steel, anadditional up-front step can be used to coat the substrate with a thinlayer of Fe to promote the growth of the diamond overlay coating. Forexample, to coat another metal, glass, ceramic or polymer substrate, avery thin layer of Fe or other catalyst is applied to the substrate toserve as an “activator” for post-deposition of nanocrystalline ormicrocrystalline diamond, as discussed above. Although both depositionoptions are permitted for most metal, glass or ceramic substrates, onlythe low temperature option seems feasible for low melting pointpolymers. This limits the thickness of nanocrystalline films that can bedeposited on polymer substrates, since deposition rates are slower atlow substrate temperatures.

Many methods are available for coating substrates with very thin layersof Fe of other catalysts, including pulsed laser deposition, atomiclayer deposition, and chemical vapor deposition. In a particularembodiment, the IDF process is used with a metalorganic feed material(e.g. iron pentacarbonyl) delivered to the substrate at a controlledrate. Thus, all the necessary steps required for coating substrates withthin films or coatings of diamond can be accomplished in a fullyintegrated IDF process, involving first depositing a very thin film ofFe or other catalyst, followed by a one- or two-step diamond-coatingtreatment to obtain the desired thickness, crystallinity, and/ortexturing. Herein, applications for diamond (and other hard materials(e.g., SiC, TiC, B4C, c-BN, and the like)) coatings to enhanceproperties and performance of fiber-reinforced composites,nanostructured composites, and laminar composites are described.

A. Fiber-reinforced Composites

Conventional fiber-reinforced composite are of generally of three maintypes: polymer matrix composites (PMCs), metal-matrix composites (MMCs),and ceramic-matrix composites (CMCs). In all three cases, fiberfabrication is a specialized operation, quite separate from actualcomposite fabrication. Hence, much effort has gone into: (1) fabricationof high specific strength fibers (e.g., carbon fibers derived frompyrolysis of polyacrylonitrile fibers), (2) methods to incorporate thefibers into the matrix phase to impart one- and two-dimensionalreinforcement, and (3) computer design of fiber weaves to achievespecific three-dimensional reinforcement of complex shapes. The fibersthemselves may be fabricated in single strands or more commonly inbundled forms (tows).

Here, the application of diamond (and other hard materials (e.g., SiC,TiC, B4C, c-BN, and the like)) coatings to available high performancefibers (e.g. C and SiC) is described to further enhance properties andperformance of 1-D, 2-D and 3-D fiber-reinforced composites. A new classof nanofiber-reinforced composites, in which the entire processingoperation does not involve fiber handling, is provided. In a particularembodiment, the fiber networks are fabricated by vapor-synthesis methods(e.g., catalytic growth of single-walled and multi-walled carbonnanotubes (CNTs)) and then infiltrated with the desired matrix phaseusing high rate IDF-deposition technology.

1. Polymer-matrix Composites

A typical polymer-matrix composite (PMC) typically comprises a highmolecular weight polymer matrix that is reinforced with high specificstrength fibers, continuous or discontinuous. Reinforcing fibersinclude, without limitation, glass, carbon, and aramid, and matrixphases including, without limitation, epoxy resin, polyester, vinylester, and polyimide. PMCs are widely used for structural applicationsbecause of their good mechanical properties, ease of manufacture, andrelatively low fabrication costs. Applications include aerospacecomponents, transportation vehicles, storage tanks, and sporting goods.

Herein, the IDF process operating in a thin film deposition mode may beused to coat bundles (tows) of glass or carbon fibers with diamond,prior to conventional PMC processing. In a particular embodiment, theIDF-deposition process involves three coordinated steps: (1) a very thincoating of Fe or other catalyst is deposited on the fibers bydecomposition of iron pentacarbonyl or other precursor; (2) a thinoverlay coating of nanocrystalline diamond is grown on the Fe-coatedfibers by low-temperature decomposition of a hydrocarbon precursor, suchas methane; and (3) a much thicker coating of textured-microcrystallinediamond is grown on the nanocrystalline diamond at a higher temperatureand at a much faster rate. Other precursor feed materials may be used,with the choice optionally being determined largely on the basis ofavailability and cost.

Well-established PMC production methods include pultrusion, prepeg, andfilament winding. All three processes lend themselves to pre-coating offibers with a thin layer of exceptionally strong and stiffnano/microcrystalline diamond, which is also much less fracture pronethat monocrystalline diamond. For example, to pre-treat carbon fibers ina pultrusion production operation, a three-step IDF coating process maybe used (see FIG. 15). After deposition of the desired thickness ofdiamond (e.g., 20% of fiber thickness), the fibers are impregnated witha thermosetting resin, pulled through a steel die to obtain the desiredshape, and then through a precision curing die to form the finalcomposite product. Almost any constant cross-sectional area product canbe manufactured, including tubes and hollow sections using centermandrels or inserted hollow cores. Similarly, the three-step IDF processmay be used for pre-treatment of fibers in prepeg, filament winding, andrelated production operations.

2. Metal-matrix Composites

A typical metal-matrix composite (MMC) comprises a lightweight metalmatrix (e.g., Al, Mg or Ti) that is reinforced with high aspect ratio orchopped fibers (e.g., carbon or silicon carbide). Suchparticle-reinforced MMCs provide high strength and stiffness, isotropicproperties, ease of near-net shape fabrication, superior thermal andelectrical properties, and affordability. Applications include, withoutlimitation, thermal management and electronic packaging, radiator panelsand battery sleeves, power semiconductor packages, microwave modules,black box enclosures, and printed circuit board heat sinks.Continuous-fiber reinforced

MMCs have not yet reached their full potential, largely because ofmanufacturing and assembly problems.

Al-matrix composites are commonly reinforced with chopped carbon fibers.Since Al reacts with carbon at elevated temperatures to form a brittle,water-soluble compound AlC₃, the carbon fibers are usually passivatedwith a thin coating of Ni. In accordance with the instant invention,carbon fibers may be first coated with a very thin layer of Fe toprovide a suitable substrate for growth of diamond, according to thetwo-step protocol described above, i.e. avoiding the third step that isabove the melting point of Al. With regard to Mg-base composites, adiamond coating is applied to the SiC fibers, prior to theirincorporation into a Mg-alloy matrix.

For higher temperature applications, a SiC-reinforced Ti matrixcomposite may be used. To mitigate harmful fiber/matrix reaction at hightemperatures, the SiC fibers are usually coated with carbon. In aparticular aspect of the invention, SiC fibers are coated with thedesired thickness of diamond prior to their incorporation into aTi-alloy matrix. A thin layer of TiC, formed by interphase-interfacereaction, enhances load transfer between fibers and matrix, thusincreasing composite strength and stiffness.

3. Ceramic-matrix Composites

A typical ceramic-matrix composite (CMC) comprises a uniformdistribution of ceramic fibers embedded in a ceramic matrix. Significantproperty advantages derived from a fiber-reinforced composite are,without limitation: (1) rupture elongations of ∥1%, (2) major increasesin fracture toughness; and (3) exceptional thermal shock resistance.Fabrication methods include chemical vapor infiltration (CVI), reactionbonding, and polymer pyrolysis. Applications include, withoutlimitation, heat shields for space vehicles, components for gas-turbineengines, brake discs, and sliding bearings.

In a particular embodiment, the CMCs are heat-resistant C/C and SiC/SiCcomposites, which are reinforced with aligned, cross-plied or wovenfibers. Chemical vapor infiltration (CVI) may be used as the fabricationmethod. To fabricate a C/C composite, for example, a woven C-fiberpreform can be exposed to an argon/methane mixture under pressure athigh temperature (e.g., 100 kPa at 1000° C.). The methane experiencesrapid thermal decomposition, depositing amorphous-like carbon on andin-between the fibers. Thus, the open space between the fibers isgradually filled, finally forming a rigid C/C composite, albeit withabout 10-15% residual porosity; the latter is subsequently reduced toabout 2-3% by liquid Si infiltration. To fabricate a SiC/SiC composite,a similar method may be used, except for the substitution of a Si-richmetalorganic compound (e.g. tetramethylsilane) as precursor material.

In this invention, infiltration of a woven C or SiC preform may beaccomplished using the three-step IDF process described above. Forexample, a woven preform may be placed in the hot zone of the burner,and subjected to IDF infiltration to obtain a uniform deposit of diamond(D) on all the fibers. The process is continued until the open spacesbetween the fibers are nearly filled, thus forming a porous D/C or D/SiCcomposite. As discussed above, this may be reduced to a few percent bypressure-assisted infiltration with liquid Si. Reaction of the liquid Siwith the diamond to form a very thin layer of SiC may occur, but thisshould not seriously impair mechanical performance. On the other hand,the presence of a significant fraction of diamond in the compositesshould dramatically increase hardness, bend strength and stiffness,without significantly reducing fracture toughness. As is well known, thelatter is determined by the degree to which fiber pull-out occurs duringfracture, which is controlled by fiber/matrix bond strength. Manyvariations of this approach can be envisioned, including infiltrationwith few-layer graphene to increase toughness of the D/C or D/SiCcomposite. This is because an advancing crack should cause delaminationof the graphene just ahead of the crack tip, thus arresting itspropagation.

For a fixed gas flow rate, the carbon-based species and/or free radicalsin the hot gas stream, which are responsible for diamond-coatingformation, progressively change in concentration as the gas streampasses through the porous preform. Hence, the deposition rate variescontinuously throughout the preform. This effect may be mitigated bystarting with a high gas flow rate (short residence time), andprogressively diminishing it. Thus, the deposition rate gradually shiftsfrom high to low at one end of the preform, while the opposite effectoccurs at the other end of the preform. Thus, a uniform diamond coatingcan be realized, thereby ensuring uniform properties in the final D/C orD/SiC composite.

A SiC/SiC composite is much less prone to high temperature oxidationthan a C/C composite, since it forms a protective scale of SiO₂. Hence,the final step in the fabrication of a D/C or D/SiC composite is theapplication of an overlay coating of SiC, also applied by IDF-depositionusing an appropriate metalorganic precursor (e.g. tetramethylsilane). Aboron-doped SiC coating is of particular interest, since it is knownthat the oxidation product (borosilicate glass) is a viscous liquid athigh temperature, so that surface cracking duet to thermal-shock isavoided.

B. Nanofiber-reinforced Composites

In the instant invention, a thin Ni grid is used upon which to grow anetwork of CNTs, thereby placing the Ni-supported CNTs just above theIDF burner and depositing a uniform layer of diamond on all the fibers.After deposition, the rigid but still highly porous diamond-coated CNTpreform is infiltrated with a thermosetting or thermoplastic polymer toform a D/CNT-reinforced PMC.

It is known that heat treating a mixture of graphite and diamondparticles in a H₂-rich gas stream can cause selective gasification ofthe graphite component. Using this for diamond-coated CNTs, theresulting diamond nanotubes (DNTs) may also be used to reinforce PMCs,owing to their exceptionally high strength and stiffness. Good loadtransfer from polmer matrix to DNTs under tensile loading is desirable.One option is to dope the diamond with B and/or N to provide activesites (dangling bonds) to promote matrix/CNT bonding. This is readilyaccomplished using the IDF process by making additions of borane (BH₃)and/or ammonia (NH₃) to the hydrocarbon feed stream.

Silicon carbide is stable in air at temperatures exceeding 1000° C.,whereas CNTs are limited to 600° C. Hence, there is growing interest indeveloping SiC nanotubes (SiCNTs) for high temperature applications.SiCNTs may be synthesized by catalytic and chemical conversion methods.Diamond-coating of SiCNT's using IDF technology enables the fabricationof a new class of D/SiCNT-reinforced CMCs for high temperaturestructural applications.

C. Laminated Composites

A typical laminated composite comprises layers of fiber-reinforcedcomposites that are bonded together to provide high in-plane strengthand stiffness, and other desirable properties, such as low thermalexpansion coefficient. Individual layers may comprise, withoutlimitation, high-strength fibers in a polymer, metal or ceramic matrix.Reinforcing fibers may comprise, without limitation, carbon, graphite,glass, or silicon carbide. Matrix materials may consist of epoxy resin,polyimide, aluminum, or titanium. Layers of different materials may beused to create hybrid composite laminates. The individual layers mayalso be designed to yield orthotropic properties (i.e. with principalproperties in orthogonal directions), or transversely isotropic (i.e.with isotropic properties in the transverse plane). Depending upon thestacking sequence of the individual layers, the laminated composite mayexhibit coupling between in-plane and out-of-plane mechanicalproperties.

Coating individual fibers of a polymer, metal or ceramic matrixcomposite with nano/microcrystalline diamond see, e.g., FIG. 16)enhances specific strength and stiffness of the composite. Thistranslates into improved properties and performance of fiber-reinforcedcomposites, irrespective of whether they are of isotropic, orthotropic,or hybrid designs. For strictly layered composites, comprising stackedarrays of different materials, coating with diamond benefits someapplications including, without limitation, personnel and vehiculararmor. Backing such layered composites with strong and toughfiber-reinforced composites, with diamond (and other hard materials(e.g., SiC, TiC, B4C, c-BN, and the like)) coatings of the fibers, isadvantageous.

Other Embodiments

The multiple inverse diffusion flame can also be used to synthesizeheterojunctions and different band gap nanomaterials. An example is theuse of a multiple inverse diffusion flame to grow metaloxide-carbide-nitride-boride nanostructures (e.g., nanowires) directlyon substrates, either alone or in combination with carbon-basedmaterials, such as CNTs. Production of non-oxide materials, such asIII-Nitrides and various carbides and borides, are also possible throughthe introduction of the appropriate precursors (either liquid or vapor).Again, the precursors do not traverse through the reaction zone. Oxidenanostructures can also be grown via hydrolysis and CO₂/CO routes due toall oxygen being consumed in the flame zones.

The invention is compatible with current technology on surfacemodification and wafer growth using CVD, but with the aforementionedadvantages and efficiencies. As such, the burner can serve as a newdelivery system for current CVD applications. The substrate can also beplaced parallel to the flow coming from the burner (not stagnation pointflow), resulting in a growing boundary layer (e.g., FIG. 11). Strategicimplementation of such an arrangement affords a favorable gradient inspecies concentration to allow continuous front-growth rather thandiscrete island-growth of graphene sheets.

The following example describes illustrative methods of practicing theinstant invention and is not intended to limit the scope of theinvention in any way.

EXAMPLE

Carbon Nanotube Synthesis

FIG. 3 shows the multiple inverse diffusion flames used for carbonnanotube (CNT) synthesis. Small individual flames are clearly seen withno soot present. Ethylene diluted with nitrogen is used as the fuelsource. A quartz cylinder encompasses the entire setup to preventoxidizer permeation from the ambient. Transition-metal substrates withdifferent compositions are inserted radially at specific locations abovethe burner to induce catalyst nanoparticle formation (from thesubstrate) and subsequent CNT growth; the substrate is held at the sameposition for 10 minutes. FIGS. 4-7 depict the growth of CNTs ondifferent metal substrates. CNTs can also be grown directly onparticles, e.g. metal-oxide spinel or carbon particles seeded withtransition metal clusters.

Graphene Synthesis

Using the multiple-inverse-diffusion-flame burner, graphene layers onnickel and copper foils have been synthesized. Since the as-receivedmetal foils invariably have a native oxide layer, it is necessary topre-treat the substrate to remove this layer. For the Cu foil, aceticacid was first used to remove any oxide layer. Afterwards, the synthesismethod consisted of treating the metal foils (Ni and Cu) in a hydrogenreducing atmosphere. For this, the burner is run initially with hydrogenas the sole fuel, which provides the hydrogen needed to reduce anyresidual oxide layer. Upon placing the metal foil above the flame for 10minutes, a hydrocarbon gas is added to the flow (FIG. 7). The metal foilis kept in the same position for an additional 10 minutes, and thenremoved from above the flame. FIGS. 8 and 9 depict the graphene layerson Ni and Cu substrates. Raman spectra are used to confirm the presenceof graphene (FIG. 10).

Diamond Synthesis

Using the multiple inverse diffusion flame burner, diamond crystals canbe nucleated, and diamond films can been grown on metal substrates. Themetal foil can be pre-treated in a hydrogen reducing atmosphere, ifnecessary. For this, the burner is run with hydrogen as the sole fuel.Upon placing the metal foil above the flame for ten minutes, hydrocarbongas is added to the flow.

A number of publications and patent documents are cited throughout theforegoing specification in order to describe the state of the art towhich this invention pertains. The entire disclosure of each of thesecitations is incorporated by reference herein.

While certain of the preferred embodiments of the present invention havebeen described and specifically exemplified above, it is not intendedthat the invention be limited to such embodiments. Various modificationsmay be made thereto without departing from the scope and spirit of thepresent invention, as set forth in the following claims.

1. A method for synthesizing a nanostructure, wherein the nanostructureis synthesized by reacting an oxidizer and a fuel in a non-premixed,multiple, inverse-diffusion-flame burner.
 2. The method of claim 1,wherein said nanostructure is selected from the group consisting ofparticles, flakes, granules, fibers, wires, films, sheets, preforms,composites, and polymers.
 3. The method of claim 1, where thenon-premixed, multiple, inverse-diffusion-flame burner comprises anarray of stabilized flames that form a uniform flat-flame front.
 4. Themethod of claim 1, where the non-premixed, multiple, inverse-diffusionflames are staged at different levels, or where inert(s), dopant(s), orother reactant(s) are introduced at level(s) different than the firstlevel of stabilized inverse-diffusion flames.
 5. The method of claim 1,wherein the oxidizer is air or O₂.
 6. The method of claim 1, wherein thefuel is a hydrocarbon, hydrogen, CO, combustible liquid, combustiblesolid fuel, or other combustible gas.
 7. The method of claim 5, whereinthe air or O₂ is replaced by another oxidizing agent.
 8. The method ofclaim 7, wherein the oxidizing agent is fluorine or bromine.
 9. Themethod of claim 1, where the nanostructure is carbon-based.
 10. Themethod of claim 9, wherein the carbon-based nanostructure is fullerene,graphene, or carbon nanotube.
 11. The method of claim 1, wherein thepyrolysis vapors exiting the non-premixed, multiple,inverse-diffusion-flame burner are directed onto substrates or particlesto form films and nanostructured coatings and preforms.
 12. The methodof claim 1, wherein the pyrolyzed species exiting the non-premixed,multiple, inverse-diffusion-flame burner: a) are quenched to generatenanostructured particles by a vapor condensation mechanism; b)infiltrates a porous preform; c) provides heating, fluidization, andprecursor loading for micron-size particles; or d) provides heating,levitation, and coating of flat substrates. 13-15. (canceled)
 16. Themethod of claim 12, wherein said substrates can translate and rotate ina continuous coating production mode.
 17. A method of synthesizingmolecular hydrogen or a syngas, wherein the molecular hydrogen or asyngas is synthesized by reacting an oxidizer and a fuel in anon-premixed, multiple, inverse-diffusion-flame burner.
 18. The methodof claim 17, wherein said fuel is selected from the group consisting ofmethane, natural gas, methanol, gasoline, diesel, JP-8, and biofuels.19. The method of claim 1, further comprising rapid quenching of a hotgas stream comprising pyrolyzed hydrocarbon, thereby generatingnanoparticles.
 20. The method of claim 19, wherein said hot gas streamfurther comprises a reactive species.
 21. The method of claim 20,wherein said reactive species is selected from the group consisting ofBH₃, H₃NBH₃, SiH₄ and (CH₃)₃SiH), thereby generating carbonnanoparticles that are enriched in B, N, Si, or mixtures thereof. 22.The method of claim 1, further comprising rapid quenching of a hot gasstream comprising pyrolyzed metalorganic or organometallic precursors,thereby generating nanoparticles of carbides, borides, nitrides,silicides, or mixtures thereof.
 23. The method of claim 1, where thepyrolysis vapors contain additives, thereby forming doped carbon-basedmaterials.
 24. The method of claim 23, wherein the additive is anitrogen species or a boron species.
 25. The method of claim 23, whereinthe dopant concentrations are sufficiently high to form nanostructuredphases.
 26. The method of claim 25, wherein nanostructured phases areC₃N₄, B₄C, or BN.
 27. The method of claim 17, wherein hydrogen issynthesized and said oxidizer is oxygen.
 28. The method of claim 27,wherein said fuel is a hydrocarbon or methane.
 29. A porous ceramichoneycomb structure that is configured for extraction of hydrogen fromreformed diesel fuel and other hydrogen-rich gases, wherein saidstructure is infiltrated with Pd alloy to form a coating.
 30. Thestructure of claim 29, wherein said Pd alloy is a Pd/Ag alloy or Pd/20Ag.
 31. The structure of claim 29, wherein said Pd alloy coating issintered to full density.
 32. The structure of claim 29, wherein the Pdalloy membrane has a nanocrystalline structure.
 33. The structure ofclaim 29, wherein the Pd alloy coating is produced by dip coating withthe application of pressure to ensure complete infiltration into theopen porous structure.
 34. A ceramic inverse-diffusion flame (IDF)burner comprising a reconfigured catalytic converter, wherein saidreconfigured catalytic converter provides separate feed streams foroxidizer and fuel.
 35. The IDF burner of claim 34, wherein the oxidizerand fuel feed streams are arranged to allow for a uniformly flat flame.36. The IDF burner of claim 34, wherein the catalyst material isselected from the group consisting of a thin section ofcatalyst-impregnated honeycomb structure, a catalyst-coated metal mesh,and a bed of catalyst-particle aggregates.
 37. The IDF burner of claim34, wherein the catalyst material is located in the hot zone of theflame.
 38. An inverse-diffusion flame (IDF) method to fabricatestrengthened composites, comprising the steps of: a) depositing Fe orother catalyst on a substrate by thermal decomposition of a metalorganicprecursor; b) depositing a thin coat of a hard material on the Fe orcatalyst-coated substrate by low-temperature thermal decomposition of ahydrocarbon precursor; and c) depositing a thicker coat of the hardmaterial on the thin-coated substrate by high-temperature thermaldecomposition of a hydrocarbon precursor.
 39. The method of claim 38,wherein the thin coating of step b) and thicker coating of step c) areof a hard material selected from the group consisting of diamond, SiC,TiC, B4C, and c-BN.
 40. The method of claim 38, wherein the metalorganicprecursor is a volatile Fe-rich compound.
 41. The method of claim 40,wherein said Fe-rich compound is iron pentcarbonyl or ferrocene.
 42. Themethod of claim 38, where thermal decomposition of the metalorganicprecursor yields a very thin deposit of nanocrystalline Fe or othercatalyst on the substrate.
 43. The method of claim 38, wherein thehydrocarbon precursor is a volatile C-rich compound.
 44. The method ofclaim 43, wherein the C-rich compound is methane or ethylene.
 45. Themethod of claim 38, wherein low-temperature thermal decomposition of thehydrocarbon precursor yields a thin deposit of nanocrystalline diamondon the Fe-coated substrate.
 46. The method of claim 38, where thenanocrystalline diamond serves as a substrate for high-temperaturethermal decomposition of the hydrocarbon precursor to develop an overlaycoating of textured microcrystalline diamond.
 47. The method of claim38, where the substrate is a fiber material.
 48. The method of claim 47,wherein the fiber material is C or SiC.
 49. The method of claim 38,wherein the substrate is a film/sheet material.
 50. The method of claim49, wherein the film/sheet material comprises a polymer, metal, orceramic.
 51. The method of claim 50, further comprising fabricatingfiber-reinforced polymer-matrix composites (PMCs), metal-matrixcomposites (MMCs), or ceramic-matrix composites (CMCs) with thediamond-coated fibers.
 52. The method of claim 49, further comprisingfabricating laminated polymer-, metal-, or ceramic-matrix compositeswith the diamond-coated film/sheet materials.
 53. The method of claim47, wherein the fiber materials are woven-fiber materials and whereinthe resultant diamond-reinforced composites contain residual porosity.54. The method of claim 53, wherein the residual porosity is filled bypressure-infiltration of a compatible liquid phase.
 55. The method ofclaim 47, wherein the fiber materials are woven-fiber materials andwherein the woven-fiber materials are infiltrated with a hard materialselected from the group consisting of diamond, SiC, TiC, B4C, and cBN byvarying the gas flow rate to obtain uniform through-thicknessdeposition.
 56. The method of claim 55, wherein the substrate is carbonnanotubes (CNTs) or silicon-carbide nanotubes (SiCNTs).
 57. The methodof claim 56, further comprising fabricating D/CNT-reinforced PMCs orD/SiCNT-reinforced CMCs from the diamond-coated CNTs or SiCNTs.
 58. Themethod of claim 56, further comprising removing the CNT component of thediamond-coated CNT by selective gasification in a hydrogen-rich gasstream, thereby forming diamond nanotubes (DNTs).
 59. The method ofclaim 58, wherein the DNTs are used to reinforce PMCs or CMCs.